The present invention relates to the field of magneto-optic recording and playback. More particularly, it relates to improvements in magneto-optic playback apparatus of the type which employs a polarization-preserving optical fiber for optically coupling a readout source and a movably-mounted objective lens for focusing readout radiation onto the data tracks of a magneto-optic recording element.
The method of optically reading magnetically recorded digital information by the use of the magneto-optic Kerr effect is well-known. Such method basically involves the steps of scan-irradiating the recording element with a continuous-wave beam of plane-polarized radiation (e.g. emanating from a c-w laser), and detecting small clockwise or counterclockwise rotations, typically on the order of 2 degrees or less, in the plane of polarization of the reflected beam. The direction of such rotation is determined by the orientation (either up or down) of the irradiated, vertically-oriented magnetic domains representing the recorded information.
In conventional magneto-optic recording systems, the recording element takes the form of a relatively thick (e.g. 1 or 2 millimeters) transparent disk which supports a relatively thin (e.g. 100 nanometers) layer of magneto-optic recording material. The magneto-optic layer may comprise any one of a variety of compounds which exhibits a relatively strong Kerr effect, and presently preferred materials include thin films of a transition metal/rare-earth alloy. During readout, the disk is rotated about its central axis while the magneto-optic layer is irradiated through the transparent support with a focused beam of plane-polarized radiation provided by a movably mounted optical head or pick-up. The source of such radiation is usually a laser diode which is mounted within the head structure and moves therewith.
In U.S. Pat. No. 4,626,679 issued on Dec. 2, 1986 in the name of Kuwayama et al. there is disclosed a variety of optical head configurations in which a laser source and a photodetector package are optically coupled to a movably mounted objective lens by one or more flexible optical fibers. This "split-head" arrangement allows the more bulky and heavier head components to remain stationary relative to the disk while only the objective lens used to focus the readout beam on the disk is moved radially relative to the disk surface. To readout the information on a magneto-optic recording element, this patent discloses an optical head comprising two fibers, one for transmitting readout radiation from the stationary source to the movably-mounted objective lens, and the other for transmitting readout radiation reflected from the recording element to a stationary photodetector package which, by means of a differential detection scheme, operates to produce a signal representing the digital information recorded.
In the commonly-assigned U.S. application Ser. No. 07/620,802, filed concurrently herewith in the names of Alan B. Marchant and entitled "MAGNETO-OPTIC READOUT USING A POLARIZATION-PRESERVING OPTICAL GUIDE", there is disclosed an improved version of the readout apparatus disclosed in the above-mentioned U.S. Pat. No. 4,626,679. Such apparatus comprises a single optical fiber for transmitting and receiving readout radiation, a 22.5 degrees phase-retardation plate positioned between the output end of the fiber and the recording element and a pair of beam-splitters positioned between the laser source and the input end of the optical fiber. The beam-splitter closer to the fiber's input end is a polarizing beam-splitter (PBS), whereas the other is not. The phase-retardation plate operates to rotate the plane of polarization of the reflected radiation by a total of 45 degrees relative to its original plane, and the optical fiber serves to convert the reflected, polarization-rotated readout radiation into its two orthogonal polarization components. One component of this reflected radiation is reflected by the polarizing beam-splitter toward a photodiode, while the other component continues toward the laser source without any substantial attenuation. The non-polarizing beam-splitter reflects a portion, e.g. 50%, of such other component to a second photodiode, and the remainder is transmitted back to the laser source. Here again, the magneto-optic readout signal is derived by a differential detection technique in which the difference in the photocurrent between the two photodiodes is measured.
While the above-described apparatus affords certain real advantages over the apparatus disclosed in U.S. Pat. No. 4,626,679, the use of a partial (i.e. non-polarizing) beam-splitter in such apparatus presents at least three disadvantages. First, the reflected radiation incident on the second photodiode (i.e. the photodiode to which the partial beam-splitter reflects radiation) is substantially lower (by about 50%) than the reflected radiation incident upon the first photodiode; therefore, the electrical gain must be higher for the second photodiode than for the first. Secondly, a substantial portion of the readout radiation is lost from the laser as it passes through the partial beam-splitter on its way to the recording element; therefore, when the same optical head is used for recording, as well as playback, less power is available for recording. Thirdly, and perhaps more importantly, a relatively large amount of readout radiation is allowed to reflect back to the laser source; this has the effect of considerably increasing the laser mode noise.